During the first two years of this project it was found that the acidic and neutral phospholipase A2 enzymes from H. haemachatus and N. naja atra snake venoms are much weaker in their pharmacological properties and lethal potency than the basic enzyme from N. nigricollis venom. Specific differences in substrate preference and membrane penetrability may account for differences in potency. Alkylation of the histidine 47 residue destroyed both pharmacological and enzymatic activity suggesting that catalytic activity is essential for pharmacological potency. During the coming year we propose to study the effects which other specific and selective amino acid modifications (lysine, arginine etc.) have on biochemical and pharmacological properties of these phospholipases. Pharmacological properties tested will include intravenous and intraventricular LD50 values and effects on action potentials and/or muscle contraction of eel electroplax, phrenic-diaphragm preparation and isolated heart muscle. Biochemical properties studied will include relative abilities to hydrolyze phospholipid substrates in mixed micelles, lipoproteins, red blood cell, electroplax, brain etc. These studies should allow us to evaluate whether the same active site is responsible for both the enzymatic and pharmacological properties of these phospholipases.